1. Field of the Invention
The present invention relates to flexible printed circuit boards and, more particularly, to a flexible printed circuit board for mounting a light emitting diode used as a backlight light source.
2. Discussion of the Background
The flexibility of flexible printed circuit boards enables them to be mounted on various kinds of products, and such flexible printed circuit boards are preferred to other products requiring flexibility in circuit wires, such as mobile phones, notebook PCs, and the like, as they permit circuit wire simplification. For example, since a flexible printed circuit board having a circuit for driving a liquid crystal display device printed thereon can be mounted in a small space, the flexible printed circuit board is commonly used for liquid crystal display devices in various products. Further, a flexible printed circuit board having light emitting diodes mounted thereon manufactured for use as a backlight light source has various applications, such as mobile communication terminals, notebook PCs, and the like.
Generally, the flexible printed circuit board has a conductive pattern between flexible insulators such as polyimide. Such flexible printed circuit boards are classified into single-sided, double-sided, and multilayer flexible printed circuit boards according to a stack structure of conductive patterns.
On the other hand, the light emitting diodes used for the backlight source are mounted on the flexible printed circuit board and are electrically connected to the conductive patterns. The light emitting diodes are disposed along an edge of a light guide plate to emit light toward the light guide plate. However, some of the light emitted from the light emitting diodes is often emitted towards an upper surface of the flexible printed circuit board and absorbed thereby. Further, some of the light having entered the light guide plate is not completely reflected by a bottom surface of the light guide plate, and can be emitted to an outside of the light guide plate there from.
In order to solve the problem that light emitted from the light emitting diode is lost by the flexible printed circuit board, a technique has been used to apply a white ink or a photoimageable solder resist (PSR) to the upper surface of the flexible printed circuit board. For example, Korean Patent Laid-open Publication No. 2006-0040863 discloses a technique of printing a white ink on the upper surface of the flexible printed circuit board to enhance transmission efficiency of light emitted from the light emitting diodes.
However, the use of the white ink or PSR entails various problems. First, since it may be difficult to uniformly apply the white ink or PSR to the flexible printed circuit board, the flexible printed circuit board has a high thickness tolerance. As a result, when assembling the flexible printed circuit board to other components, such as the light guide plate and the like, the thickness tolerance of the flexible printed circuit board can cause assembly defects between the components.
Furthermore, when applying and curing the white ink or PSR on the circuit board, a difference in concentration or thickness of the ink on the circuit board can cause generation of bubbles or clumping, thereby causing scratches or cracks during an assembly process or in use, and can also cause yellowing by heat.